The invention relates to a method and device for checking test specimens comprising digital-to-analog converters and/or analog-to-digital converters or telecommunications transmission links which contain such converters or are connected in series thereto.
The known advantages of digital message transmission and switching, regenerability of the signals as desired, the possibility of processing and editing them by means of modern computer technology, as well as the availability of highly developed and economical digital modules and also including the significant structural simplification of telecommunications systems have led to the universally observable introduction of digital message systems. The recent availability of extremely broad-band transmission channels in the form of light waveguides accommodates the tendency toward digitization of the electrical message industry.
Since the original information in most cases, for example in telephone technology and in image transmission of half-tone masters, are analog values, they must be converted into digital signals with the assistance of suitable matching devices. These matching devices, which must carry out the analog-to-digital conversion at the inputs of the systems and the digital-to-analog conversion at their outputs, are presently the most problematical modules of the digital communications system. This pertains both to their function as well as to the device-oriented expense embodied by them within a system, making up approximately half of the overall hardware expense in, for example, digital telephone switching systems. It is obvious that every advance in the development and manufacture of cost-favorable converter modules which meet the strict demands raised has a positive influence on the economic feasibility of the overall system.
Checking the analog-to-digital and/or digital-to-analog converter generally realized as micro-electronic circuits or, respectively, the modules containing said converters is accorded special significance for these reasons. The checking should provide not only precise information concerning the qualification of the respectively tested specimen but, rather, should also provide conclusions concerning systematic manufacturing deficiencies.
Checking digital converter modules with the assistance of traditional transmission test positions requires comparatively long measuring and testing time which, among other things, depends on the transient response of the existing filters so that, in particular, measuring the quantization distortion is relatively involved.
Test installations are known ("Fairchild Telecom Test Adapter", Brochure 11/80 10 M 129000) and incorporated herein by reference wherein the signal samples gained by means of time-discrete sampling are subjected to a fast Fourier transformation and, by so doing, are transferred from the time range into the frequency range so that the spectral components are available for further investigation. The investigation of the signal samples ensues with suitable mathematical methods by means of a computer.
Other modern methods for checking converter modules are disclosed in the periodical "Der Elektroniker", 6-181, pages 56 through 58 in the article "An Integrated Analog/Digital Test System, 1981 Proceedings Electronic Components Conference", pp. 202-206; and "Real Time Measuring Method for CODEC Characteristics Using Digital Signal Processing", FUJITSU Scientific & Technical Journal, June 1981, pp. 15-33.